Technical Abstract:
Failure to remove the blanket of residue generated during green-cane harvesting and certain glyphosate ripener application regimes have independently been shown to reduce yields of the subsequent ratoon crop of Louisiana’s leading variety LCP 85-384. The objectives of this experiment were to determine residual glyphosate effects in ratoon crops of three newly released varieties Ho 95-988, HoCP 96-540, and L 97-128 following an application the previous year and to investigate if the stress associated with the application of glyphosate is compounded by the presence of post-harvest residue at the start of the growing season. The combination of a ripener application of glyphosate to the first-ratoon crop and the failure to remove the residue blanket generated during the harvest of the first-ratoon crop reduced the sugar yields of the subsequent second-ratoon crop by 21% (3150 kg/ha) compared to treatments where glyphosate was not applied and the residue was removed within two weeks of harvest. On the other hand, where ripener was not used, post-harvest residue retention lowered sugar yields by 6% (920 kg/ha) compared to plots where the residue was removed. This sugar loss was primarily due to lower cane tonnage (103.3 Mg/ha) where both ripener and residue stresses were imposed verses just the residue stress (116.3 Mg/ha). Where residue was removed by burning, ripener application did not reduce sugar yields compared to where ripener was not used. To optimize ratoon crop yields, Louisiana farmers should remove post-harvest residue as soon after harvest as possible especially if ripener was applied the previous season. In areas where residue cannot be removed growers should consider not applying a ripener because of the compounded effects of these two stresses.